1. Field of the Invention
The present invention relates to an induction heating apparatus employing an inverter power source for effecting a heating process by induction heating, a heat fixing apparatus for heat fixing an unfixed toner image formed on a sheet, to such sheet utilizing such induction heating apparatus, and an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus provided with such induction heat fixing apparatus.
2. Related Background Art
In an image forming apparatus, a fixing apparatus of a heat roller type has been widely employed in order to fix an unfixed image (toner image) of desired image information, formed by a direct method or an indirect method on a recording material (a transfer sheet, an electrofax sheet, an electrostatic recording paper, an OHP sheet, a printing paper or a formatted paper) in a process unit of a suitable image forming process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process, as a permanent fixed image onto such recording material. In recent years, an apparatus of belt (film) heating type has also been commercialized for achieving a quick start or an energy saving. Also there is proposed an apparatus of electromagnetic induction heating system.
Among these, the present invention can be advantageously applied to the fixing apparatus of the induction heating type. In the induction heating fixing apparatus, an alternating magnetic flux (high frequency magnetic field) generated by magnetic field generating means is applied to an electromagnetic induction heat generating member, serving as a heat generating member, thereby inducing an eddy current therein and generating a Joule's heat by the resistance thereof, and the unfixed toner image is fixed by such generated heat to the surface of the recording material as a permanent fixed image.
Japanese Utility Model Application Laid-open No. 51-109739 discloses an induction heating fixing apparatus in which a current is induced in a fixing roller by a magnetic flux thereby generating a Joule's heat. Such apparatus can directly heat the fixing roller by utilizing generation of an induction current, thereby achieving a fixing process of a higher efficiency than in a fixing apparatus of heat roller type utilizing a halogen lamp as the heat source.
In a prior induction heating apparatus provided with an inverter power source, which supplies an exciting coil with a current by turning on and off a rectified output of a commercial power supply thereby executing induction heating of a heated member to a predetermined temperature, a power control signal is generated based on a comparison of a detected temperature of the heated member and a target temperature, and the temperature control is achieved by regulating a current supply interval of the excitation coil according to thus generated power control signal thereby controlling the amount of heat generation.
In the above-described configuration, since the voltage of the commercial power supply is supplied, without stabilization, directly to a load of a macroscopically constant resistance by on/off operation of the switch, the input electric power increases almost proportionally to the square of the input voltage. Therefore, in the above-explained temperature control method, the maximum supplied power varies significantly by the input voltage and the fluctuation in the start-up time becomes larger than in the halogen heater, in case of employing the commercial power supply showing a large voltage fluctuation range.
In order to prevent the change in the maximum supplied power resulting from the fluctuation in the input voltage, Japanese Patent Application Laid-open No. 9-120221 proposes an induction heating apparatus which detects the power supply voltage and executes a control of regulating the current supply interval according to a result of comparison with a reference voltage, thereby providing a substantially constant maximum supplied power regardless of a fluctuation in the power supply voltage.
Also, in order to correct not only the influence of an external fluctuation factor such as the power supply voltage but also the influence of an internal factor or a load variation, such as a rush current at a cold start-up operation, Japanese Patent Application Laid-open No. 10-301442 proposes an induction heating apparatus which detects also a current flowing in the load, and calculates a supplied power from the result of such detection and that of the power supply voltage detection means, thereby setting the maximum supplied power.
However, in the method proposed in Japanese Patent Application Laid-open No. 10-301442, as it becomes necessary to detect the power supply voltage and the current in the circuit of the primary side and to transmit these values for processing to the circuit of the secondary side where a temperature control unit is provided, there are required expensive components such as a photocoupler or a transformer in plural units, whereby the cost becomes inevitably high.
Also in any of the aforementioned related technologies, there is always set a constant maximum supplied power over a voltage range of the commercial power supply. However, as shown in FIG. 14, the upper limit of the usable current (1503, 1504) for the rated current value varies depending on the regional safety regulations, so that the usable power (1505, 1507) varies for each regional voltage range, and a upper limit line (1506, 1508) of the power usable in the induction heating apparatus, obtained by subtracting the maximum power consumption in a low-voltage power source becomes uneven as illustrated. Consequently, none of the aforementioned related technologies is applicable to a product designed for plural regions.
Stated differently, in the method of setting the maximum power, the maximum power supply has to be set at the lowest limit (1509) of the upper limit line (1506, 1508) of the usable power, so that the maximum power under a low voltage condition, which is least efficient for the warm-up time, is uniquely selected for all the voltage ranges.